bpf: fix bpf_skb_pull_data documentation

Fix documentation for bpf_skb_pull_data() helper for
when len == 0.

Fixes: fa15601a ("bpf: add documentation for eBPF helpers (33-41)")
Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Acked-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/r/20220715193800.3940070-1-joannelkoong@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>